1. Field of the Invention
This invention relates to the field of waste heat recovery systems, and more particularly to waste heat recovery systems for recovering waste heat contained in exhaust gases for subsequent beneficial use, such as the pre-heating inlet air in an inlet plenum.
2. Prior Art
A substantial quantity of heat energy is generated as a by-product of many chemical and industrial processes. In many cases, this heat is exhausted into the atmosphere through exhaust stacks and flues because the cost of its recovery has been greater than could be economically justified. This heat is known in the field as "waste heat" because it is, in fact, all too often wasted energy. Perhaps, twenty (20) years ago and earlier, an industrial society could afford to waste energy on a massive scale because the cost of one million BTU's of energy was only about 8 cents. Today, however, the cost of one million BTU's of energy is about $2.00. Thus, there exists today a great economic incentive to recover the waste heat of chemical and industrial processes and to use it beneficially in the process; e.g., to preheat the inlet combustion air.
Waste heat recovery systems are known to the prior art. However, the systems of the prior art have one or more significant shortcomings and limitations. One waste heat recovery system well known and commonly used in the prior art utilizes the so-called Ljungstrom heat exchanger. The Ljungstrom heat exchanger is a regenerative heat exchanger in that it includes a regenerator drum rotatably mounted in a housing divided into separate compartments through which the hot exhaust gases and the cool gases to be heated flow. The drum, driven by an electric motor, has a capacity for heat absorption and release. As the drum rotates, it absorbs waste heat from the hot exhaust gases in one compartment and gives up the heat to the cooler gases in the other compartment.
The Ljungstrom heat exchanger imposes several severe limitations upon any waste heat recovery system in which it is utilized. In the first place, the regenerator drum must be relatively large in order for sufficient waste heat to be recovered. The large drum, in turn, requires the use of large exhaust and inlet compartments and associated ducts, the latter often 6 feet or larger in diameter. Secondly, by virtue of the use of a drum as the basic heat exchange medium, the two compartments of the housing must be located adjacent to one another. Thus, if the gases heated by the drum are to be used at a location remote from the location of the source of the waste heat gases, ducting must be provided between the exchanger and such remote point. Further, a blower of sufficient capacity must also be provided in order to force the heated gases to flow to the remote point of utilization. Thus, such regenerator drum systems suffer from the disadvantages of higher cost (due to the ducting and blower capacity required) and from the fact that they require relatively large installations which, together with the associated ducting, tie up much valuable property in a non-productive manner. For the foregoing reasons, a waste heat recovery system utilizing a Ljungstrom heat exchanger may prove to be economically unfeasible in some applications. In addition, such systems are typically more difficult to install than systems which use a fluid heat transfer medium, such as the present invention. In the latter case, 4 inch pipes are typically used in lieu of 6 foot or larger ducts. Another disadvantage of waste heat recovery systems which employ a Ljungstrom heat exchanger is that they are limited to transferring waste heat from hot exhaust gases to cooler combustion air.
Heat exchange apparatuses and methods are known in the prior art. Such apparatuses are generally used to transfer "process heat", as distinguished from waste heat, from one point in the process to another. Many such heat exchangers utilize a liquid heat transfer medium. However, the temperature and other conditions present in a waste heat recovery application are typically far more severe than those encountered in applications wherein process heat is being transferred. Thus, a reliable and economical waste heat recovery system cannot be constructed by simply utilizing the heat exchanger apparatuses and methods of the prior art, suitable for the transfer to process heat, to solve a waste heat recovery problem.
U.S. Pat. No. 3,623,549, issued to Horace L. Smith, is an example of a prior art heat exchanger utilizing a plurality of heat transfer liquids. Smith's invention transfers heat from a gas at one location to a cooler gas at a second location which may be considerably removed from the first location. Smith discloses the use of at least two independent flow circuits through which different heat transfer liquids flow. Each flow circuit comprises a pair of interconnected finned tube type heat exchangers. The first heat exchanger of each circuit is located in a duct through which the hot gas flows, while the second is located in a duct through which the cool gas flows.
While U.S. Pat. No. 3,623,549 teaches the use of a suitable heat transfer liquid in closed flow circuits for the transfer of heat from one point to a second remote point, it applies such teachings to a general and relatively simple application of hot and cool gases flowing in two separate ducts. Smith's invention does not address itself to the particular conditions typically found in waste heat recovery applications where, for example, the temperatures and pressures at various points are critical parameters which must be controlled. To illustrate this point, the following two temperature constraints on waste heat recovery systems are cited: (i) the temperature of the heat transfer liquid must not reach a level which could damage the pumping means typically used in the flow circuit; (ii) the temperature of the exhaust gases must not be permitted to drop to a temperature at which some of the gases may begin to condense onto the heat exchanger located in the exhaust stack, because such condensation would cause corrosion of the exchanger. If condensation of the exhaust gases is not prevented or substantially mitigated, the heat exchanger in the stack would have to be replaced periodically, thereby causing expense and down time in the operation of the process. Thus, the teachings of Smith are inadequate for the waste heat recovery applications for which the present invention is advantageously suited.
There have been attempts in the prior art to apply heat exchange apparatuses, utilizing a fluid as a heat transfer medium, for the recovery and transfer of waste heat. U.S. Pat. No. 2,699,758 issued to David Dalin, is an example of one such attempt. Dalin discloses an apparatus for improving combustion in the furnaces of steam boilers by preheating the combustion air, in two stages, to a relatively high temperature by using the flue gases as a source of heat for this purpose. He teaches the use of water as a first heat transfer medium in a first stage of waste heat recovery and superheated steam as the medium of heat transfer in the second stage thereof.
Unlike Smith, Dalin discloses the uses of some temperature and pressure control means; e.g., (i) an economizer 19 to insure a definite temperature differential between the two zones of the flue passage at which the heat exchangers draw their heat; (ii) a thermostatically controlled valve 34 which controls flow through a bypass pipe 33; and (iii) a thermally response control element 35 which controls the opening of the valve 34. However, the invention of Dalin suffers from one of the major shortcomings of the prior art at the time of its invention (circa 1950); namely, the unavailability of heat transfer liquids suitable for the high temperatures encountered in waste heat recovery applications. Many of the heat transfer liquids of the prior art flash off at the high temperatures typically encountered in an exhaust stack, thereby creating a fire hazard; others tend to corrode the piping means through which they flow. While water and steam, as heat transfer mediums, do not flash off or cause as much corrosion as other liquids, they have their own disadvantages. Water, since it boils at 100C., is inherently limited with respect to the amount of heat it can absorb without changing phase. On the other hand, steam, especially superheated steam, introduces the obvious disadvantages of high pressure; for example, severe design requirements with respect to the structural strength of the heat exchange apparatus and associated piping, and (ii) maintenance problems with respect to the detection and repair of leaks.
Still another disadvantage of the Dalin system, attributable to its use of superheated steam as the heat transfer medium, is the limitation that the latter imposes with respect to the distance between the flue passage (or exhaust stack) and the place to which the waste heat is to be delivered. If the distance is great enough, the continuing loss of heat through the conducting pipes may cause the superheated steam to condense to water. As a result of the heat transfer medium being in two phases within the flow circuit (i.e., steam and condensed water), its flow becomes non-uniform and difficult to regulate. If the flow of the heat transfer medium cannot be readily regulated, the control of critical temperatures within the system becomes more difficult, if not impossible.
U.S. Pat. Nos. 3,405,509 and 3,405,769 disclose means for recovering waste heat in the exhaust stack of fired oil field equipment. The invention disclosed in U.S. Pat. No. 3,405,509 is limited in that it uses, as the heat transfer medium, the very oil well product fluids, (e.g., a mix of oil and water) which are being processed. U.S. Pat. No. 3,405,759 likewise teaches the use of the process liquid as the heat transfer medium. However, the latter patent also teaches the use of a separate heat transfer fluid contained in a source separate from the process fluids; in the latter connection, however, the patent teaches the use of water and steam as the separate heat transfer fluid, both of which have the disadvantages and limitations described above with reference to U.S. Pat. No. 2,699,758 (Dalin).
Most recently a system has been introduced which utilizes a first heat exchanger in the stack or flue, a second heat exchanger at the waste heat utilization site, such as by way of example the inlet air plenum to a set of burners, with a suitable liquid heat transfer medium carrying the waste heat therebetween. This system with its associated control apparatus provided for efficient waste heat recovery. However, installation of such systems could be time consuming, and the down time cost while the system was being installed could be on the same order as the cost of the waste heat recovery system, or higher. Aside from the fact that the first heat exchanger was actually mounted in the stack or flue, thereby requiring substantial cutting, welding and plumbing while the system is shut down, many furnace systems, particularly those fabricated since the second world war, do not have adequate structure in the stack or flue to support the heat exchanger and associated hardware, and accordingly the entire stack has to be reinforced from ground level to provide adequate structure for the required load. This is time consuming, and can lead to excessive down time, making the cost of the waste heat recovery system excessive and deterring the more immediate application of such system.